So there's a scientific correlation that's been found (if someone can find the studies that back this that'd be great...I can't remember where I saw it at) between dreams, memory, and recollection. The connection goes like this:

Your subconscious takes the events of your life and stores the important (or unresolved) ones as memories. Your dreams are the background effort to resolves the unresolved things...
Dreams that work toward internalizing events <- Waking world activity -> memories

Your subconscious, however, still has to handle the dream events as an experience, so when things get internalized and stored as memories, this weird thing happens where dreams are treated as real life events.

There is no concrete proof regarding why this happens, but the logic follows that:
Your subconscious filters "real life" events from "fictional events" based on personal involvement (if it happened to you or if you have the capability to confirm the results of the event in the real world). This is why we can remember reading a book, but not have memories as if we lived the book's story.

Dreams, while not physically real in the waking world, do still effect us directly. For this reason, memories of dreams are stored as real life events - because the experience directly involves the dreamer, instead of being something the dreamer experienced by proxy like in the case of any storytelling media.

-- Dreams may not be 'real', but their significance to the dreamer makes them just as valid as real world memories, if not moreso... --